Polytetrafluoroethylene (PTFE) layers have been used for the manufacture of various types of intracorporeal devices, such as vascular grafts. Such vascular grafts may be used to replace, reinforce, or bypass a diseased or injured body lumen. One conventional method of manufacturing “expanded” PTFE layers is described in U.S. Pat. No. 3,953,566 by Gore. In the methods described therein, a PTFE paste is formed by combining a PTFE resin and a lubricant. The PTFE paste may be extruded. After the lubricant is removed from the extruded paste, the PTFE article is stretched to create a porous, high strength PTFE article. The expanded PTFE layer is characterized by a porous, open microstructure that has nodes interconnected by fibrils.
Such an expansion process increases the volume of the PTFE layer by increasing the porosity, decreasing the density and increasing the internodal distance between adjacent nodes in the microstructure while not significantly affecting the thickness of the PTFE layer. As such, the conventional methods expand the PTFE layer and impart a porosity and permeability while only providing a negligible reduction in a thickness of the PTFE layer. In situations where a thin PTFE layer, and specifically, a thin PTFE layer having a low fluid permeability is needed, conventional PTFE layers are largely unsatisfactory due to the porosity and highly permeable nature of the expanded PTFE layer.
Therefore, what have been needed are improved PTFE layers and improved methods for manufacturing the PTFE layers. In particular, it would be desirable to have thin PTFE layers that have a controllable permeability to fluids (gases, liquids or both). It may also be desirable to have such thin PTFE layers that have a high degree of limpness and suppleness to allow mechanical manipulation or strain of such a PTFE layer without significant recoil or spring back.